Method and system for in situ formation of gas-phase compounds

ABSTRACT

A system and method for providing intermediate reactive species to a reaction chamber are disclosed. The system includes an intermediate reactive species formation chamber fluidly coupled to the reaction chamber to provide intermediate reactive species to the reaction chamber. A pressure control device can be used to control an operating pressure of the intermediate reactive species formation chamber, and a heater can be used to heat the intermediate reactive species formation chamber to a desired temperature.

FIELD OF DISCLOSURE

The present disclosure generally relates to gas-phase reactors andsystems. More particularly, the disclosure relates to gas-phase systemsand methods capable of in situ formation of gas-phase compounds that canbe used as precursors or reactants in a downstream reactor.

BACKGROUND OF THE DISCLOSURE

Gas-phase processes are used for a variety of applications, such aschemical vapor deposition processes to deposit material onto asubstrate, gas-phase etching processes to remove material from asubstrate or a reactor, gas-phase cleaning processes to clean asubstrate or reactor, and gas-phase treatment processes to treat asurface of a substrate or a reactor. Precursors for gas-phase processesare generally selected according to a material to be deposited, etched,cleaned, or treated; i.e., the precursors are generally selected toprovide desired gas-phase reactants. However, other factors are oftenused to select between more than one precursor that might be suitablefor a particular application. For example, a reactivity or selectivityof a precursor may be a factor in the selection of the precursor.Another consideration for selecting a precursor is the stability of theprecursor—e.g., does the precursor break down into other compoundsbefore the precursor has a chance to take part in a desired reaction.Yet further considerations may include vapor pressure of the precursor,toxicity of the precursor, availability of the precursor, and cost ofthe precursor. Thus, a precursor that might have desirable properties,such as higher selectivity, reactivity, and/or provide more uniformdeposition, etch, or treatment, may not be selected for a particularapplication, because the precursor is relatively expensive, has anundesirable vapor pressure, and/or is toxic.

Remote or direct plasma systems may be used to create activated orenergized species from a precursor, where the energized species are morereactive than the precursor for a given reactor temperature. Remoteplasma systems generally form a plasma upstream of a reaction chamber,and direct plasma systems generally form a plasma within a reactionchamber, where a substrate is often in or adjacent to the plasma. Remoteplasma systems may be advantageous over direct plasma systems for someapplications, because the remote plasma systems do not form a plasmadirectly over a surface of a substrate. As a result, surface damage to asubstrate that might otherwise occur in a direct plasma reactor can bereduced or eliminated using a remote plasma. However, remote plasmaactivated species from many precursors are relatively short lived andrecombine or react with other components before the activated speciesenter the reaction chamber or reach a desired area of a substrate (e.g.,a lower portion of a trench formed on a surface of the substrate and/oran outer perimeter of the substrate). Using a direct plasma allows theactivated species to form within the reaction chamber, but the activatedspecies may still recombine or otherwise become inactivated prior toreacting desired areas on a substrate.

Accordingly, improved methods and systems for forming reactive speciesrelatively close to a substrate without causing unwanted substratedamage, wherein the reactive species may be relatively stable aredesired.

SUMMARY OF THE DISCLOSURE

Various embodiments of the present disclosure provide improved methodsand systems for forming intermediate reactive species (also referred toherein as compounds) from one or more precursors. The intermediatereactive species can be suitable for use in various gas-phase processes,such as chemical vapor deposition processes (including plasma-enhancedchemical vapor deposition processes), gas-phase etching processes(including plasma-enhanced gas-phase etching processes), gas-phasecleaning (including plasma-enhanced cleaning processes), and gas-phasetreatment processes (including plasma-enhanced gas-phase treatmentprocesses).

Exemplary methods and systems can be used to form intermediate reactivespecies near a reaction chamber, where, for example, the intermediatereactive species might be considered a desirable reactant but anundesirable (e.g., toxic or unstable) precursor source. The methods andsystems can be used to provide a steady-state source of desired chemicalspecies, such as particular reactants to a reaction chamber of areactor. A plasma system that is remote from the reaction chamber can beused to facilitate formation of the desired intermediate reactivespecies.

In accordance with various embodiments of the disclosure, a gas-phasereactor system includes a reactor comprising a reaction chamber, anintermediate reactive species formation chamber fluidly coupled to thereaction chamber, a first gas source fluidly coupled to the intermediatereactive species formation chamber, and a pressure control deviceinterposed between the intermediate reactive species formation chamberand the reaction chamber. The pressure control device can be used tocontrol an operating pressure of the intermediate reactive speciesformation chamber. The reactor may be, for example, a chemical vapordeposition reactor, an atomic layer deposition reactor, an etch reactor,a clean reactor, or a treatment reactor, any of which can include director remote plasma apparatus. In accordance with various aspects of theseembodiments, the system further includes a controller coupled to thepressure control device to maintain a desired operating pressure of theintermediate reactive species formation chamber. In accordance withfurther aspects, the system includes one or more flow control units(e.g., mass flow controllers) to control flow rates of one or moregasses to the intermediate reactive species formation chamber. Exemplarysystems can also include a heater to heat one or more gasses and/or theintermediate reactive species formation chamber to a desiredtemperature—e.g., to a temperature of about 50° C. to about 200° C. Inaccordance with further aspects, the pressure control device is aclosed-loop pressure controller that controls a gas pressure upstream ofthe pressure control device. And, in accordance with yet additionalaspects, the system further comprises an integrated inlet manifold blockbetween the intermediate reactive species formation chamber and thereactor. The intermediate reactive species formation chamber can includea catalyst to facilitate formation of desired intermediate reactivespecies.

In accordance with additional exemplary embodiments of the invention, amethod of forming intermediate reactive species for use in a reactionchamber of a reactor includes the steps of providing a first gas to anintermediate reactive species formation chamber, controlling a pressurewithin the intermediate reactive species formation chamber, and formingintermediate reactive species within the intermediate reactive speciesformation chamber. Exemplary methods in accordance with theseembodiments can be used for depositing material onto a surface of asubstrate, etching a material on a surface of a substrate, cleaning asurface of a substrate, treating a surface of a substrate, depositingmaterial onto a surface of a reaction chamber, etching a surface of areaction chamber, treating a surface of the reaction chamber, and/orcleaning a surface of the reaction chamber. In accordance with variousaspects of these embodiments, the method additionally includes providinga second gas to the intermediate reactive species formation chamber. Inaccordance with further aspects, the step of controlling a pressure ofthe intermediate reactive species formation chamber comprises using aclosed-loop upstream pressure controller. In accordance with furtheraspects, the method includes a step of forming a plasma in a remoteplasma unit, which can be selected from the group consisting of aninductively coupled plasma unit and a microwave unit. In accordance withyet further aspects, a method includes controlling a valve between theintermediate reactive species formation chamber and the reactionchamber. In accordance with additional aspects, a method includes a stepof heating the intermediate reactive species formation chamber to adesired temperature—e.g., to a temperature of about 50° C. to about 200°C.

In accordance with yet additional embodiments of the invention, aplasma-enhanced reactor system, such as a plasma-enhanced chemical vapordeposition reactor (e.g., a plasma-enhanced atomic layer depositionreactor) system, a plasma-enhanced etch reactor system, aplasma-enhanced clean reactor system, or a plasma-enhanced treatmentreactor system, includes a reactor comprising a reaction chamber, anintermediate reactive species formation chamber fluidly coupled to thereaction chamber, a remote plasma unit fluidly coupled to theintermediate reactive species formation chamber, a first gas sourcecoupled to the intermediate reactive species formation chamber, and apressure control device in fluid communication with and interposedbetween the intermediate reactive species formation chamber and thereaction chamber. The pressure control device controls an operatingpressure of the intermediate reactive species formation chamber. Inaccordance with various aspects of these embodiments, the system furtherincludes a controller coupled to the pressure control device to maintaina desired operating pressure of the intermediate reactive speciesformation chamber. In accordance with further aspects, the systemincludes one or more flow control units to control flow rates of one ormore gasses to the remote plasma unit and/or the intermediate reactivespecies formation chamber. In accordance with further aspects, thepressure control device is a closed-loop pressure controller thatcontrols a gas pressure upstream of the pressure control device. And, inaccordance with yet additional aspects, the system further comprises anintegrated inlet manifold block between the remote intermediate reactivespecies formation chamber and the reactor. In accordance with yetadditional aspects of these embodiments, the intermediate reactivespecies formation chamber includes a catalyst—e.g., to facilitateformation of desired intermediate species. And, in accordance withadditional aspects, the system includes a heater—e.g., to heat theintermediate reactive species formation chamber to a temperature ofabout 50° C. to about 200° C.

Both the foregoing summary and the following detailed description areexemplary and explanatory only and are not restrictive of the disclosureor the claimed invention.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete understanding of exemplary embodiments of the presentdisclosure can be derived by referring to the detailed description andclaims when considered in connection with the following illustrativefigure.

FIG. 1 illustrates a gas-phase reactor system in accordance withexemplary embodiments of the disclosure.

It will be appreciated that elements in the figure are illustrated forsimplicity and clarity and have not necessarily been drawn to scale. Forexample, the dimensions of some of the elements in the figure may beexaggerated relative to other elements to help to improve theunderstanding of illustrated embodiments of the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE DISCLOSURE

The description of exemplary embodiments provided below is merelyexemplary and is intended for purposes of illustration only; thefollowing description is not intended to limit the scope of thedisclosure or the claims. Moreover, recitation of multiple embodimentshaving stated features is not intended to exclude other embodimentshaving additional features or other embodiments incorporating differentcombinations of the stated features.

Exemplary methods and systems include use of an intermediate reactivespecies formation chamber to form intermediate reactive species. Theintermediate reactive species can be used in subsequent reactions, suchas deposition, etch, clean, and/or treatment reactions in a downstreamreaction chamber.

FIG. 1 illustrates a gas-phase reactor system 100 in accordance withexemplary embodiments of the disclosure. System 100 includes a reactor102, including a reaction chamber 104, a substrate holder 106, a gasdistribution system 108, an intermediate reactive species formationchamber 148, a remote plasma unit 110, a vacuum source 112, a firstreactant gas source 114, a second reactant gas source 116, one or moreadditional reactant gas source(es) 118, purge gas sources 120, 122, 124,one or more flow control units 126-136, a pressure control device 140,and a controller 142 coupled to pressure control device 140. System 100can also include a heater 150, and/or an integrated inlet manifold block144. Although not illustrated, system 100 may additionally includethermal excitation for one or more reactants.

Reactor 102 may be used to deposit material onto a surface of asubstrate 146, etch material from a surface of substrate 146, clean asurface of substrate 146, treat a surface of substrate 146, depositmaterial onto a surface within reaction chamber 104, clean a surfacewithin reaction chamber 104, etch a surface within reaction chamber 104,and/or treat a surface within reaction chamber 104. Reactor 102 can be astandalone reactor or part of a cluster tool. Further, reactor 102 canbe dedicated to deposition, etch, clean, or treatment processes asdescribed herein, or reactor 102 may be used for multipleprocesses—e.g., for any combination of deposition, etch, clean, andtreatment processes. By way of examples, reactor 102 may include areactor typically used for chemical vapor deposition, such asplasma-enhanced chemical vapor deposition (PECVD) and/or plasma-enhancedatomic layer deposition (PEALD) processing.

Substrate holder 106 is designed to hold substrate or workpiece 146 inplace during processing. In accordance with various exemplaryembodiments, reactor 102 includes direct plasma apparatus; in this casesubstrate holder 106 can form part of a direct plasma circuit.Additionally or alternatively, substrate holder 106 may be heated,cooled, or be at ambient process temperature during processing. By wayof example, substrate holder 106 can be heated during substrate 146processing, such that reactor 102 is operated in a cold-wall,hot-substrate configuration.

Although gas distribution system 108 is illustrated in block form, gasdistribution system 108 may be relatively complex and be designed to mixgas (e.g., vapor) from reactant sources 114, 116, 118 intermediatespecies formation chamber 148, and/or carrier/purge gases from one ormore sources 120, 122, 124 prior to distributing the gas mixture toreaction chamber 104. Further, system 108 may be configured to providevertical (as illustrated) or horizontal flow of gasses to the chamber104. An exemplary gas distribution system is described in U.S. Pat. No.8,152,922 to Schmidt et al., issued Apr. 10, 2012, entitled “Gas Mixerand Manifold Assembly for ALD Reactor,” the contents of which are herebyincorporated herein by reference, to the extent the contents do notconflict with the present disclosure. By way of example, distributionsystem 108 includes a showerhead gas distribution system.

Remote plasma unit 110 is a remote plasma device, which is capable offorming a plasma. By way of particular examples, remote plasma unit 110can be an inductively coupled plasma unit or a microwave remote plasmaunit. In the illustrated example remote plasma unit 110 can be used tocreate reactive or excited species for use in intermediate reactivespecies formation chamber 148 and/or reactor 102. Although system 100 isillustrated with remote plasma unit 110, systems in accordance withother exemplary embodiments of the disclosure do not include a remoteplasma unit. In addition to or as an alternative to using remote plasmaunit 110 to form excited species, system 100 can include anotherexcitation source, such as a thermal or hot filament source, a microwavesource, or the like.

Vacuum source 112 can include any suitable vacuum source capable ofproviding a desired pressure in reaction chamber 104. Vacuum source 112may include, for example, a dry vacuum pump alone or in combination witha turbo molecular pump.

Reactant gas sources or precursors 114, 116, and 118 can each includeone or more gases, or materials that become gaseous, that are used indeposition, etch, clean, or treatment processes. Exemplary gas sourcesinclude nitrogen trifluoride (NF₃). ammonia (NH₃), water vapor (H₂O).hydrogen peroxide (H₂O₂), MMH (mono methyl hydrazine), UDMH(unsymmetrical dimethyl hydrazine), O₂/H₂, N₂/H₂, and H₂S. Althoughillustrated with three reactant gas sources 114-118, systems inaccordance with the disclosure can include any suitable number ofreactant sources.

As noted above, system 100 can be used to form intermediate reactivespecies from one or more precursors from a gas source such as one ormore of gas sources 114-118. Because system 100 can form intermediatereactive species, precursors (e.g. from gas sources 114-118) can haverelatively desirable precursor qualities—e.g., be relatively safe,inexpensive, etc., while the intermediate reactive species may have moredesirable reactant qualities—e.g., be relatively reactive and providerelatively even deposition or etch characteristics across a surface of asubstrate and/or within a reaction chamber. Exemplary intermediatereactive species formed from ammonia include, for example, ammoniumfluoride, hydrazine (N₂H₄), NH₂, which is relatively unstable, anddiazene (N₂H₂). Both hydrazine and diazene are considered toxic and arenot typically used in vapor deposition processes. However, bothhydrazine and diazene have superior properties when forming nitridematerials using vapor deposition processing. The present inventionallows for the safe, easy formation of these intermediate reactivespecies. Similarly, OH— intermediate reactive species from H₂O may beformed using the system described herein. Additional intermediatereactive species include H₂O₂ (peroxide), HO₂, NH, NH₄F (e.g., fromexcited NF₃ species/Ar introduced via remote plasma unit 110 and NH₃introduced to (e.g., heated) intermediate reactive species formationchamber 148), N₂H, and HS (e.g., from H₂ 5), trisilophosphines, andexcited species thereof. The terms “activated” and “excited” are usedinterchangeably herein.

In the context of reactor etching, treating, or cleaning, theintermediate reactants that are formed can be used to etch, treat, orclean reactor parts, such as a fore line, that might otherwise not becleaned, treated, or etched with less stable reactants.

Purge gas sources 120-124 include one or more gases, or materials thatbecome gaseous, that are relatively unreactive in reactor 102. Exemplarypurge gasses include nitrogen, argon, helium, and any combinationsthereof. Although illustrated with three purge gas sources, system inaccordance with the present disclosure can include any suitable numberof purge gas sources. Further one or more purge gas sources can provideone or more carrier gasses and/or system 100 can include additionalcarrier gas sources to provide a carrier gas to be mixed with one ormore gases from a reactant source.

Flow controllers 126-136 can include any suitable device for controllinggas flow. For example, flow controllers 124-132 can be mass flowcontrollers.

Intermediate reactive species formation chamber 148 allows formation ofdesired intermediate reactive species, which can then be introduced intoreaction chamber 104—e.g., in a steady-state manner. A pressure withinintermediate reactive species formation chamber 148 can be controlledusing pressure control device 140. System 100 can also include a pressersensor 152 to measure a pressure within intermediate reactive speciesformation chamber 148. In the illustrated example, pressure controldevice and pressure sensor 152 are coupled to controller 142 to control(e.g., closed-loop control) a pressure within intermediate reactivespecies formation chamber 148. Pressure control device 140 can includeany suitable device that controls an upstream pressure. By way ofexample, pressure control device 140 is an active (e.g., closed-loop)pressure controller, such as MKS model 640A pressure controller.Alternatively, pressure control device can include a throttle valve.System 100 can be configured to pulse intermediate reactive species fromintermediate reactive species formation chamber 148 to reaction chamber104 using pressure control device 140 or other suitable valve.

A pressure within intermediate reactive species formation chamber 148can be controlled independently from the pressure within reactionchamber 104. A pressure within intermediate reactive species formationchamber 148 can vary according to application. By way of examples, apressure in intermediate reactive species formation chamber 148 canrange from about 10 milliTorr to about 10 Torr.

Heater 150 can be used to heat intermediate reactive species formationchamber 148 to a desired temperature. Heater 150 can be configured toindependently control a temperature of intermediate reactive speciesformation chamber 148—e.g., independent from a temperature withinreaction chamber 104. Exemplary systems can control both a temperatureand a pressure within intermediate reactive species formation chamber148. Heater 150 can be a (e.g., resistive) jacket heater, a built-inheater, a radiant heater, or the like. In accordance with illustratedexamples of the disclosure, heater 150 is configured to heatintermediate reactive species formation chamber 148 to a temperature ofabout 50° C. to about 200° C., or about 75° C. to about 175° C., orabout 100° C. to about 150° C. Although not illustrated one or sourcegas lines (e.g., lines 131, 135, 137) and/or one or more purge gas lines(e.g., lines 127, 129, 133) can be heated to facilitate obtaining and/orretaining a desired temperature within intermediate reactive speciesformation chamber 148 and/or reaction chamber 104.

As illustrated, intermediate reactive species formation chamber 148includes an inlet 154. Inlet 154 can include a first region 156 that hasa larger cross-sectional opening relative to a downstream section 158.Further, inlet 154 can be tapered, e.g., in a frusto-conical shape, toprovide angled walls. The angled walls can promote desired mixing ofreactants within intermediate reactive species formation chamber 148.One or more reactants can be introduced to intermediate reactive speciesformation chamber 148 at or near inlet 154 to allow the reactantsadditional time to mix and react to form desired compounds/specieswithin intermediate reactive species formation chamber 148.

Intermediate reactive species formation chamber 148 can be formed from avariety of materials, such as stainless steel or a Hastelloy® alloy.Intermediate reactive species formation chamber 148 can also include acatalyst within (e.g., coated on a surface or as a packed bed)intermediate reactive species formation chamber 148. The catalyst can beused to facilitate formation of one or more desired intermediatereactive species. For example, in the case when ammonia is used to makehydrazine, the catalyst may include iron, manganese oxide (MgO), ortitanium oxide (TiO2). Other suitable catalytic materials include noblemetals, such as platinum, palladium, and rhodium. Additionally oralternatively intermediate reactive species formation chamber 148 caninclude a liner, such as a quartz liner.

Optional integrated inlet manifold block 140 is designed to receive anddistribute one or more gasses to reaction chamber 104. An exemplaryintegrated inlet manifold block 140 is disclosed in U.S. Pat. No.7,918,938 to Provencher et al., issued Apr. 5, 2011, entitled “HighTemperature ALD Inlet Manifold,” the contents of which are herebyincorporated herein by reference, to the extent the contents do notconflict with the present disclosure.

Although exemplary embodiments of the present disclosure are set forthherein, it should be appreciated that the disclosure is not so limited.For example, although the methods and reactor systems are described inconnection with various specific configurations, the disclosure is notnecessarily limited to these examples. Various modifications,variations, and enhancements of the exemplary systems and methods setforth herein may be made without departing from the spirit and scope ofthe present disclosure.

The subject matter of the present disclosure includes all novel andnonobvious combinations and subcombinations of the various systems,components, and configurations, and other features, functions, acts,and/or properties disclosed herein, as well as any and all equivalentsthereof.

What is claimed is:
 1. A gas-phase reactor system comprising: a reactorcomprising a reaction chamber; an intermediate reactive speciesformation chamber fluidly coupled to the reaction chamber; a first gassource fluidly coupled to the intermediate reactive species formationchamber, wherein the first gas source comprises a precursor forintermediate reactive species; and a pressure control device in fluidcommunication with the intermediate reactive species formation chamberand interposed between the intermediate reactive species formationchamber and the reaction chamber, wherein the pressure control devicecontrols an operating pressure of the intermediate reactive speciesformation chamber.
 2. The gas-phase reactor system of claim 1, furthercomprising a remote plasma unit coupled to the intermediate reactivespecies formation chamber.
 3. The gas-phase reactor system of claim 1,further comprising a controller coupled to the pressure control deviceto control a pressure within the intermediate reactive species formationchamber.
 4. The gas-phase reactor system of claim 1, further comprisinga flow control unit coupled to the first gas source.
 5. The gas-phasereactor system of claim 1, wherein the reactor is selected from thegroup consisting of a chemical vapor deposition reactor, an atomic layerdeposition reactor, an etch reactor, a clean reactor, and a treatmentreactor.
 6. The gas-phase reactor system of claim 1, wherein thepressure control device is a closed-loop pressure control device.
 7. Thegas-phase reactor system of claim 1, further comprising an integratedinlet manifold block.
 8. The gas-phase reactor system of claim 1,wherein the intermediate reactive species formation chamber comprises acatalyst.
 9. The gas-phase reactor system of claim 1, further comprisinga heater capable of heating the intermediate reactive species formationchamber to a temperature between about 50° C. and about 200° C.
 10. Thegas-phase reactor system of claim 1, wherein the first gas sourcecomprises one or more or NH₃, NF₃, H₂O,H₂O₂, MMH (mono methylhydrazine), UDMH (unsymmetrical dimethyl hydrazine), O₂/H₂, N₂/H₂, andH₂S.
 11. A method of forming intermediate reactive species for use in areaction chamber of a reactor, the method comprising the steps of:providing a first gas to an intermediate reactive species formationchamber; controlling a pressure within the intermediate reactive speciesformation chamber; and forming intermediate reactive species within theintermediate reactive species formation chamber.
 12. The method offorming intermediate reactive species for use in a reaction chamber of areactor of claim 11, further comprising providing a remote plasma unitcoupled to the intermediate reactive species formation chamber.
 13. Themethod of forming intermediate reactive species for use in a reactionchamber of a reactor of claim 12, further comprising the steps of:providing a second gas to the remote plasma unit; and providing anexcited species to the intermediate reactive species formation chamber.14. The method of forming intermediate reactive species for use in areaction chamber of a reactor of claim 12, wherein the step of providinga remote plasma unit comprises providing a plasma unit selected from thegroup consisting of an inductively coupled plasma unit and a microwaveunit.
 15. The method of forming intermediate reactive species for use ina reaction chamber of a reactor of claim 11, wherein the step ofcontrolling a pressure within the intermediate reactive speciesformation chamber comprises using a closed-loop pressure controller. 16.The method of forming intermediate reactive species for use in areaction chamber of a reactor of claim 11, wherein the step of formingintermediate reactive species comprises controlling a valve between theintermediate reactive species formation chamber and the reactionchamber.
 17. The method of forming intermediate reactive species for usein a reaction chamber of a reactor of claim 11, further comprising oneor more steps selected from the group consisting of: depositing materialonto a surface of a substrate, etching a material on a surface of asubstrate, cleaning a surface of a substrate, treating a surface of asubstrate, depositing material onto a surface of the reaction chamber,etching material from a surface of the reaction chamber, treating asurface of the reaction chamber, and cleaning a surface of the reactionchamber.
 18. A gas-phase reactor system comprising: a reactor comprisinga reaction chamber; an intermediate reactive species formation chamberfluidly coupled to the reaction chamber; a first reactant source coupledto the intermediate reactive species formation chamber, wherein thefirst reactant source is a precursor for intermediate reactive species;a pressure control device in fluid communication with and interposedbetween the intermediate reactive species formation chamber and thereaction chamber, wherein the pressure control device controls anoperating pressure of the intermediate reactive species formationchamber; and a heater to heat the intermediate reactive speciesformation chamber to a temperature of about 50° C. to about 200° C. 19.The gas-phase reactor system of claim 18, further comprising acontroller coupled to the pressure control device.
 20. The gas-phasereactor system of claim 18, wherein the intermediate reactive speciesformation chamber comprises a catalyst.